The increasing dominance of Facebook, and the fading of other sites like MySpace and Classmates.com, makes me wonder about the concept of one ruling social network. Human beings, after all, naturally belong to a variety of networks based on different contexts like shared interests, work, school, geography and the like. So it seems logical that there is room for specialized or niche social networks oriented around those specific groups or activities.

Three things make me think so:

A handful of social networks delivering specialized functions have big audiences.

Facebook’s effort to add specialization to its network is immature.

Other networks can harness social services through open APIs — if they’re careful.

Facebook’s flaws give niche social networks a chance.

Social networks enable their users to share information and experiences through conventions like personal profiles, linked contacts and real-time feeds of communications and status updates. But companies have gained large audiences using social networking techniques to deliver focused communications without the help of actual social networks like Facebook or MySpace. Examples include Twitter (175 million micro-bloggers and readers), LinkedIn (90 million users of professional and career-oriented contacts), Foursquare (6 million users doing location-based check-ins) and Instagram (2 million mobile photo sharers).

Meanwhile, Facebook’s own attempt to deliver niche networks within its own general-purpose network is Groups. The company revamped its Groups service last October. The service lets users create subsets of their contact networks in the hopes they will create things like family photo albums and local soccer clubs. But Groups isolates communications and activities within a group, rather than filtering them. That means users must duplicate their efforts if they want to share across groups. And Groups isn’t open to marketers or advertisers, even though activities on the service are often more desirable for campaign targeting.

Managing an open API ecosystem has its risks.

At the same time Facebook’s solution to niche social networks is incomplete, it is providing companies and apps developers APIs and services to build their own, like BranchOut‘s career network within Facebook. To be sure, it can be risky to depend on another company’s platform. Last week, Twitter locked out UberMedia and TwapperKeeper for using unapproved URL shorteners, and for archiving and re-licensing data without an official Twitter deal. Last year, Facebook and Google blocked each other from harvesting contacts, and Apple had to remove Facebook connectivity from its Ping social network due to the “onerous terms” Facebook wanted as compensation for Apple’s presumed heavy usage. The takeaway: Specialist social networks should negotiate before implementing.

Hunch co-founder Chris Dixon suggests that developers should trust platform companies who are open with their product roadmaps, or at least exhibit predictable strategies. Platforms without established revenues, such as Twitter or Foursquare are less transparent. Compared with them, for instance, Facebook’s business model is clear: It sells advertising and collects a percentage of spending on virtual goods via its Credits system, and usually only bullies big competitors.

Companies should use Facebook and the others for customer acquisition, then ease off technology dependencies. For example, use Facebook log-in initially, and then migrate users to an OAuth-based process later by suggesting they update their password for security. After importing social graph data, build up your own database of friend connections by tracking specialized communications.